LTC3875_15 Datasheet, PDF (24/44 Pages) Linear Technology – Dual, 2-Phase, Synchronous Controller with Low Value DCR Sensing and Temperature Compensation

LTC3875
Applications Information
a resistor divider and applied to the TK/SS pin. Therefore,
the voltage ramp rate on this pin is determined by the
ramp rate of the other supply’s voltage. Note that the small
soft-start capacitor charging current is always flowing,
producing a small offset error. To minimize this error, select
the tracking resistive divider value to be small enough to
make this error negligible. In order to track down another
channel or supply after the soft-start phase expires, the
LTC3875 is forced into continuous mode of operation
as soon as V_FB is below the undervoltage threshold of
0.55V regardless of the setting on the MODE/PLLIN pin.
However, the LTC3875 should always be set in forced
continuous mode tracking down when there is no load.
After TK/SS drops below 0.1V, its channel will operate in
discontinuous mode.
The LTC3875 allows the user to program how its output
ramps up and down by means of the TK/SS pins. Through
these pins, the output can be set up to either coincidentally
or ratiometrically track another supply’s output, as shown
in Figure 9. In the following discussions, V_OUT1 refers to
the LTC3875’s output 1 as a master channel and V_OUT2
refers to the LTC3875’s output 2 as a slave channel. In
practice, though, either phase can be used as the master.
To implement the coincident tracking in Figure 9a, con-
nect an additional resistive divider to V_OUT1 and connect
its midpoint to the TK/SS pin of the slave channel. The
ratio of this divider should be the same as that of the
slave channel’s feedback divider shown in Figure 10a. In
this tracking mode, V_OUT1 must be set higher than V_OUT2.
To implement the ratiometric tracking in Figure 10b, the
ratio of the V_OUT2 divider should be exactly the same as
the master channel’s feedback divider shown in Figure 9b.
By selecting different resistors, the LTC3875 can achieve
different modes of tracking including the two in Figure 9.
So which mode should be programmed? While either
mode in Figure 9 satisfies most practical applications,
some trade-offs exist. The ratiometric mode saves a pair
of resistors, but the coincident mode offers better output
regulation. When the master channel’s output experiences
dynamic excursion (under load transient, for example),
V_OUT1
V_OUT1
V_OUT2
V_OUT2
24
TIME
(9a) Coincident Tracking
TIME
3875 F09
(9b) Ratiometric Tracking
Figure 9. Two Different Modes of Output Voltage Tracking
V_OUT1
TO
TK/SS2
PIN
R3 R1
TO
V_OSNS1+
PIN
TO
V_OSNS2+
PIN
R4 R2
V_OUT2
R3
R4
(10a) Coincident Tracking Setup
V_OUT1
TO
TK/SS2
PIN
R1
TO
TO
V_OSNS1+
V_OSNS2+
PIN
PIN
R2
V_OUT2
R3
R4
3875 F10
(10b) Ratiometric Tracking Setup
Figure 10. Setup for Coincident and Ratiometric Tracking
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